What is about the man who declared himself king of his own island???
I think you are referring to the ex-British Army guy who declared the independence of the Principality of Sealand, which was on a disused World War 2 sea fort about 6 miles off the coast of Essex. He basically landed on there and declared an independent nation. It’s more than 3 miles off the mainland of Great Britain, i.e. in international waters, so technically he could, but I don’t think that it’s officially recognised by other countries though, it’s more of a commercial gimmick now.